The present invention relates generally to a rotating locking or positioning mechanism and method for locking or positioning a plate or sheet-like member by placing it in tension. More particularly, the present invention relates to a self-tensioning device for securing a printing plate to a printing cylinder in a rotary printing press.
It is common practice in the printing industry to use flexible metallic plastic or paper printing plates having a raised or planographic image on one side thereof attached to a printing cylinder in a rotary printing press. The raised image on the printing plate is transferred to the paper as the printing cylinder rotates and the paper is moved through the rotary printing press.
Printing plates are typically changed relatively often whenever a different image is desired. Because the rotary printing press must be shut down in order to change printing plates, printing plates must be easily changeable so as to reduce costly downtime.
As explained in U.S. Pat. No. 4,332,197 to Dulin, efforts to devise low cost printing plates have resulted in printing plates which are dimensionally unstable when exposed to different humidity and temperature levels. The resulting variation in printing plate dimensions creates a need for a self-tensioning printing plate locking mechanism to maintain tension on the printing plate to closely fit the periphery of the press cylinder.
Because the press operator must change the printing plates frequently, it is important that the locking mechanism be simple to operate. Moreover, the operators must work in very close spaces with great possibility of injury to the operator or marring the printing due to excessive handling. Thus, simplicity of the operation of a locking mechanism for rotary printing presses is paramount.
Because printing plate locking mechanisms are often difficult to access for repair, it is important that they require infrequent repair to minimize expensive downtime. Thus, it is desirable to construct such a mechanism with as few moving parts as possible so as to minimize breakdowns.
A number of printing plate locking mechanisms are found in the art. These mechanisms are relatively complicated mechanically and expensive to build and maintain.
Thus, there has been a need in the field of printing plate locking devices for such a device which is easily operated, inexpensive to construct and maintain, and which provides constant tension on printing plates of varying lengths.